An exhaust gas sensor (e.g., exhaust oxygen sensor) may be positioned in an exhaust system of a vehicle and operated to provide indications of various exhaust gas constituents. In one example, the exhaust gas sensor may be used to detect an air-fuel ratio of exhaust gas exhausted from an internal combustion engine of the vehicle. The exhaust gas sensor readings may then be used to control operation of the internal combustion engine to propel the vehicle. In another example, outputs of the exhaust gas sensor may be used to estimate a water content in the exhaust gas. Water content estimated using the exhaust gas oxygen sensor may be used to infer an ambient humidity during engine operation. Further still, the water content may be used to infer an alcohol content of a fuel burned in the engine. Under select conditions, the exhaust gas sensor may be operated as a variable voltage (VVs) oxygen sensor in order to more accurately determine exhaust water content. When operating in the VVs mode, a reference voltage of the exhaust gas sensor is increased from a lower, base voltage (e.g., approximately 450 mv) to a higher, target voltage (e.g., in a range of 900-1100 mV). In some examples, the higher, target voltage may be a voltage at which water molecules are partially or fully dissociated at the oxygen sensor while the base voltage is a voltage at which water molecules are not dissociated at the sensor.
However, the inventors herein have recognized potential issues with operating the exhaust gas sensor in the VVs mode. As one example, air-fuel estimates with the exhaust gas sensor may be invalid when the reference voltage is increased above the base voltage since the oxygen sensor is no longer stoichiometric. For example, at higher reference voltages, the sensor dissociates water vapor and carbon dioxide which contribute to the oxygen concentration represented in the pumping current output by the exhaust gas sensor. Since water vapor and carbon dioxide change with ambient humidity and ethanol concentration in the fuel, and these parameters are unknown, traditional pumping current to air-fuel ratio transfer functions are not accurate at elevated reference voltages. As a result, the vehicle may have to operate in open loop fuel control which may negatively impact emissions, fuel economy, and drivability.
In one example, the issues described above may be at least partially addressed by a method for an engine comprising: responsive to a first condition comprising a reference voltage of a first exhaust oxygen sensor operating in variable voltage mode increasing above a threshold voltage, determining a change in an output of the first exhaust oxygen sensor corresponding to the increase in the reference voltage, correcting the output of the first oxygen sensor based on the output change, and adjusting engine operation based on the corrected output.
In another example, a method may comprise, during operation of a combustion engine in a closed loop control mode based on air-fuel ratio, correcting an output of a first exhaust oxygen sensor operating in a variable voltage mode, and determining the air-fuel ratio based on the corrected output of the first exhaust oxygen sensor.
In this way, the technical effect of preserving the accuracy of air-fuel estimates based on the exhaust gas sensor, and maintaining closed loop fuel control of the engine even when the exhaust oxygen sensor is operating VVS mode can be achieved, thereby reducing engine emissions, increasing fuel economy, and increasing vehicle drivability.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.